For certain applications, it is desirable to have a magnetic loop sensor, tuned by the addition of capacitance to resonate in the frequency range of 1-1000 MHz. In order to detect very weak magnetic fields such a sensor must generate extremely low levels of noise and consequently must have extremely low resistance and hence low loss. The lowest loss sensors are made of superconductor materials. Until recently, all superconductor materials had to be cooled below 30K to operate as superconductors. This requirement added significantly to the cost and complexity of systems which relied on these materials. The sensor design described here is one appropriate to high temperature superconductors, i.e., superconductor materials whose critical temperature is higher than 30K. This latter class of materials, also known as cuprates, oxide superconductors and perovskites, is better suited to use in thin film form than in bulk forms. This physico-chemical difference necessitates new device and circuit designs to make these materials useful in superconducting applications. The design can also be fabricated using conventional superconductors, like niobium, which are available in thin film form.
This invention consists of a multi-turn spiral coil (having inductance L) with an internal distributed interdigital capacitor (having capacitance C). The device operates in a self-resonant mode. For certain magnetic resonance imaging (MRI) applications, resonant frequencies of approximately 5 MHz and sufficiently low resistance that the coil has a resonant quality factor (Q) of not less than 10.sup.4, and even as high as 10.sup.6, are desired. The low resistances implied by these high values of Q ensure that the coil's internally generated thermal noise will be less than the noise generated by other noise sources within the imaging system, such as the tissue or object being imaged or the preamplifier coupled to the coil. To achieve such high Q, it is necessary that the equivalent series resistance of the LC resonator be less that approximately 100 .mu..OMEGA. to 1 m.OMEGA.. Such low resistance is achieved by the use of superconducting thin-film metallization in both the coil and the capacitor. A key advantage of this approach is that the sensor can be produced with a single superconductive film, and as a result it is more easily and reproducibly manufactured.